During my first night of training, my (then) manager preempted her tutorial on closing a group for the evening by saying, “There is only one way to do this, and Eric can taste if his shots came from a group that was insufficiently cleaned the night before.’’

The next time I closed a group, I repeated the comment to the barista who was training me, and he seemed to be of the opinion that there was little discernible difference, and, thus, no reason for anyone to suspect a group had gone poorly cleaned.  These two initial encounters sparked a curiosity in me, so I put it to the scientific method.

My aim was to examine the differences between a shot of espresso from a freshly cleaned group and one from a group that had been tainted by the morning rush.  In the middle of the mid-day backflush, after I had thoroughly scrubbed one portafilter and its complementary group head, I extracted a pair of eighteen-second, "clean" shots.  Conveniently, the two "dirty" shots also poured for eighteen seconds - this allowed me to factor time out of my analysis.
 
From the first, piping-hot sip of golden crema, it becomes apparent there is no struggle to distinguishing the "clean" and "dirty" shots from one another.  To be plain, both taste deserving of their namesake; the clean espresso is reddish-brown and has the strong, crisp aroma of citrus and bitter fruits, while the scent of the dirty, muddy-brown shot is sour and metallic.  The rancid oils in the portafilter from the morning use gave the dirty espresso a bitter, abrasive taste of burnt metal that causes an initial gustatory response that I fondly refer to as "flavor recoil — in other words, it's difficult not to automatically wrinkle your face in distaste.  In contrast, the clean shot is smooth and bright with a full, nutty flavor, high drinkability, and lingering hints of the fruits apparent in its aroma.  Without much competition, the shot of espresso from the freshly cleaned group is superior.

I also did a little research and discovered that the essential oils in coffee (which may collect in the holes of the basket, on the internal brass, or the spout of the portafilter) become rancid in about 45 minutes.  Most sources actually recommend a quick soak and scrub of each portafilter every 30-60 minutes in addition to the mid-day backflush and full cleaning we practice each evening.  If you're intrigued, I encourage you to repeat this experiment in your own store and taste the difference for yourself - it may force you to re-examine how often and how thoroughly you scrub that brass!